1. Technical Field
The present disclosure relates to an optical isolator.
2. Description of the Related Art
Recently, optical fiber lasers are attracting a lot of attention. This is because optical fiber lasers have many advantages such as good beam forming characteristics, high power efficiency, output power stability, cost-efficiency and lightness, etc.
The optical fiber laser may include: a gain fiber such as an optical fiber doped with rare-earth ions used as a gain medium; an optical fiber grating acting as a reflection mirror of a laser cavity; a pump light source such as a laser diode; a seed light source or seed laser; and a optical fiber delivery that delivers an output laser beam to a desired destination.
Typically, a laser beam emitted from a laser can be reflected to return to the laser for several factors. This causes problems such as distortion in the laser beam or damage to optical components. To overcome such problems, an optical isolator is employed that transmits a laser beam emitted from a laser and blocks a laser beam returning back to the laser.
Examples of the optical isolator include a polarization dependent isolator that operates only in a particular polarization direction and a polarization independent isolator that operates regardless of a polarization direction. For more details, reference is made to U.S. Pat. No. 4,548,478 and the paper “Polarization-independent in-line optical isolator with lens free configuration,” published in Journal of Lightwave Technology, Vol. 10, p. 1839, 1992. A typical optical isolator essentially includes a polarizer and a Faraday rotator, and optionally includes a wave plate, a polarization rotator and a silt. There is a problem in that such optical components may not operate properly at a desired wavelength due to wavelength-specific characteristics, especially dispersion characteristics of the rotator and polarizer, so that optical isolation may fail. This may result in breaking laser optical components such as the pump light source, the seed light source, etc., and also optical components inside the optical isolator.
Moreover, optical fiber lasers have high power above several kW and generate ultrashort output pulses, such that they are becoming more complicated. A master oscillator power amplifier (MOPA) system may be one example for the complicated structure. Furthermore, various kinds of light leakage may occur in an optical fiber laser to thereby damage laser optical components or deteriorate the performance thereof. Examples of light leakage may include oscillating laser light and pump light. Even when the relative amount of a light leakage is small, for example, a leakage of oscillating laser light that is not blocked by a laser cavity may easily damage a pump light source and in turn break an optical isolator and cause the optical isolator to deteriorate as the power of the optical fiber laser increases. For another example, pump light that is not completely absorbed by a gain fiber and leaks may break other laser optical components, especially a seed light source, or deteriorate the performance thereof.
FIG. 9 is a diagram for illustrating a configuration of an optical fiber laser system in the related art.
Referring to FIG. 9, pump light emitted from a plurality of laser diodes acting as pump light sources passes through a pump combiner, and is incident on a laser cavity consisting of a pair of fiber Bragg gratings (FBGs) and a gain fiber doped with rare-earth ions therebetween. In the laser cavity, laser light is amplified by receiving the energy of the pump light and oscillates at a particular oscillation wavelength. Finally, a laser light is emitted via an output end disposed on the right hand of the laser cavity, and is ready to be used. Generally, the left one of the fiber Bragg gratings, which is closer to the pump light source, has a reflectivity between 99% and 99.9% at laser oscillation wavelength, such that majority of laser light oscillating in the laser cavity is reflected toward the right hand thereof. However, some of the oscillating laser light may pass through the left one of the fiber Bragg gratings to leak toward the pump light source. Such light leakage is ignorable in a laser system having a low power. However, in a laser system having a high power, light leakage may break a pump light source and other laser optical components.
FIG. 10 is a diagram for illustrating another optical fiber laser system in the related art.
Referring to FIG. 10, a laser system includes: a seed light source used as a light source for laser; an optical isolator operating at the wavelength of the seed light source; and an amplification unit consisting of pump light sources each including a plurality of laser diodes, pump combiners connected to the respective pump light sources, and a gain fiber disposed between the pump combiners. Some of pump light generated in a pump light source disposed on the right side of the gain fiber may not be completely absorbed by the gain optical fiber and thus fail to be converted into oscillating laser light. Accordingly, some of the pump light may leak toward the seed light source. Generally, an optical isolator disposed at the output end of the seed light source operates at the wavelength of the seed light source but does not operate at the wavelength of the pump light. As a result, the seed light source is likely to be damaged. For example, for an optical fiber laser system at 1,550 nm implemented using an optical fiber doped with erbium (Er) ions, the wavelength of the seed light source may be 1,550 nm and the wavelength of the pump wavelength may be 980 nm. In this example, an optical isolator for a seed beam operates at 1,550 nm, and thus leacked pump light at 980 nm cannot be blocked effectively.
To overcome this problem, Korean Patent Application No. 10-2012-0032097 discloses an optical fiber for protecting a laser pump light source, in which ions for absorbing an oscillating laser light is doped in an optical fiber, based on the idea that the absorptivity of the optical fiber is higher in the oscillation wavelength range than in the pump wavelength range. However, the approach requires disposing a specially-treated optical fiber on the optical path additionally, and thus is not desirable in terms of cost, processing time and manufacturing difficulty. In addition, according to the approach, only the pump light source below a short-wavelength range (below 990 nm) can be protected from an oscillating laser light (above 1,030 nm).
In addition, in order to ensure that pump light sources are protected and components are prevented from being degraded and damaged, it may be contemplated to apply a filter for blocking an oscillating laser light on the pump light source. However, this requires applying a coating process on every pump light source, and is thus undesirable in terms of cost, processing time and manufacturing difficulty. In addition, this technique has a limited maximum efficiency of blocking light leakage. Accordingly, what is required is a method for blocking light leakage simply and efficiently.